Victoria E. Hamilton on tue 11 jan 05
Petra,
Make sure your shelves are pristine. Then spread either some dry EPK or
some dry silica over the shelf before you set your slab on the shelf.
Either of these substances will act sort of like tiny ball bearings and
allow for the expansion and contraction of the slab during heating, firing
and cooling.
Good luck.
Victoria Hamilton
Millennia Antica Pottery
Seattle, WA
-----Original Message-----
From: Clayart [mailto:CLAYART@LSV.CERAMICS.ORG] On Behalf Of Petra Hahn
Sent: Tuesday, January 11, 2005 07:45
To: CLAYART@LSV.CERAMICS.ORG
Subject: Why do slabs crack?
Hi Clayarters,
I tried to make big structured slabs (about 2,75 in.), which should imitate
stone slabs.
But in firing they get a few short, thin radial cracks. If not in bisque
firing, then in stoneware firing.
I guess it happens, because the slab can't move on the kiln shelf in the
firing. Is that right?
How can I avoid these cracks?
They even appear in smaller slabs, too.
I hope, someone can give me a hint.
Greetings from Germany
Petra
____________________________________________________________________________
__
Send postings to clayart@lsv.ceramics.org
You may look at the archives for the list or change your subscription
settings from http://www.ceramics.org/clayart/
Moderator of the list is Mel Jacobson who may be reached at
melpots@pclink.com.
Petra Hahn on tue 11 jan 05
Hi Clayarters,
I tried to make big structured slabs (about 2,75 in.), which should imitate
stone slabs.
But in firing they get a few short, thin radial cracks. If not in bisque
firing, then in stoneware firing.
I guess it happens, because the slab can't move on the kiln shelf in the
firing. Is that right?
How can I avoid these cracks?
They even appear in smaller slabs, too.
I hope, someone can give me a hint.
Greetings from Germany
Petra
Snail Scott on tue 11 jan 05
At 04:44 PM 1/11/2005 +0100, you wrote:
>...in firing they get a few short, thin radial cracks. If not in bisque
>firing, then in stoneware firing.
>I guess it happens, because the slab can't move on the kiln shelf in the
>firing...
If it IS a problem deriving from friction with
the shelf, a little sand or grog underneath the
slab will act like ball bearings and allow the
clay to move freely.
It may be a firing-related crack, though, due to
the outer edges of the clay heating up faster
than the middle. Try firing slower than you do
now, and see if that helps. Longer firings take
more time and resources, but not as much as
remaking the work would!
-Snail
Ann Brink on tue 11 jan 05
Perhaps the edges are ready to shrink before the middle is, so stress cracks
appear. One suggestion I've heard is to lay stilts next to all the edges,
to equalize heating & cooling. Let us know if this works-
Ann Brink in Lompoc CA
----- Original Message -----
From: "Petra Hahn"
To:
Sent: Tuesday, January 11, 2005 7:44 AM
Subject: Why do slabs crack?
> Hi Clayarters,
>
> I tried to make big structured slabs (about 2,75 in.), which should
imitate
> stone slabs.
> But in firing they get a few short, thin radial cracks. If not in bisque
> firing, then in stoneware firing.
> I guess it happens, because the slab can't move on the kiln shelf in the
> firing. Is that right?
> How can I avoid these cracks?
> They even appear in smaller slabs, too.
> I hope, someone can give me a hint.
>
> Greetings from Germany
> Petra
>
>
____________________________________________________________________________
__
> Send postings to clayart@lsv.ceramics.org
>
> You may look at the archives for the list or change your subscription
> settings from http://www.ceramics.org/clayart/
>
> Moderator of the list is Mel Jacobson who may be reached at
melpots@pclink.com.
Darnie Sizemore on tue 11 jan 05
Petra,
There might not be enough grog in your clay to allow
the water to escape.
Depending on how you create the slab, the particles
might be streched to far and need to be compacted
again. I use a rib to smooth the clay out, I start at
the edges and smooth into the center. This will help
to compact the clay.
Another suggestion is to allow the slabs to dry on a
piece of newspaper, the paper will move with the clay
and allow it to shrink without any resistance.
And finally, you might want to put some grog on the
kiln shelf as you fire. It will also alow the clay to
shrink without resistance during the firing.
Good luck!
Darnie
Detroit, MI
Freezing rain today
50 degrees tomorrow
--- Petra Hahn wrote:
> Hi Clayarters,
>
> I tried to make big structured slabs (about 2,75
> in.), which should imitate
> stone slabs.
> But in firing they get a few short, thin radial
> cracks. If not in bisque
> firing, then in stoneware firing.
> I guess it happens, because the slab can't move on
> the kiln shelf in the
> firing. Is that right?
> How can I avoid these cracks?
> They even appear in smaller slabs, too.
> I hope, someone can give me a hint.
>
> Greetings from Germany
> Petra
>
>
______________________________________________________________________________
> Send postings to clayart@lsv.ceramics.org
>
> You may look at the archives for the list or change
> your subscription
> settings from http://www.ceramics.org/clayart/
>
> Moderator of the list is Mel Jacobson who may be
> reached at melpots@pclink.com.
>
__________________________________
Do you Yahoo!?
Take Yahoo! Mail with you! Get it on your mobile phone.
http://mobile.yahoo.com/maildemo
Michael Wendt on tue 11 jan 05
Petra,
You can do a number of things to reduce cracking in large slabs.
First, consider trying different clay bodies. Not all perform equally
well for this task. If the slabs are to be subjected to freeze-thaw
conditions, test the clay for stability first.
Second, if none of the clays you have available work, you may
need to alter the clay by the addition of grog (fired clay) to reduce
cracking.
Consider also wedging using the lamination method.
A piece is cut in half, stacked, then slammed onto the table
to join the two. That is cut in half and joined and each time
this occurs, the number of layers doubles. By 30 doublings,
there are over a billion layers and the slab is then rolled
out with the laminations parallel to the slab roller table.
Such tiles can be made huge without warping.
Firing can be done on edge for bisque.
see: http://www.wendtpottery.com/clayart.htm for a photo
of a stack.
Once you survive the bisque,
Center the tiles on the shelves and ring them with saggar
blocks made of kiln posts, fire bricks or you can make
your own saggars by hand or with an extruder. The saggars
slow the rate of heating and cooling.
It also helps if you alternate your layers in the kiln.
Start with a tall layer in the bottom, then a tile layer,
then a tall layer , then a tile layer and finish with a tall
layer. Trying to fire all tiles is hard because the shelves
prevent direct radiation from reaching very far in so
the heat is mainly by conduction and thus slower and
less uniform if you fire fast.
One final note, fire slower than normal through red heat to
avoid the quartz inversion problem.
Regards,
Michael Wendt
Wendt Pottery
2729 Clearwater Ave
Lewiston, Idaho 83501
USA
wendtpot@lewiston.com
www.wendtpottery.com
Petra wrote:
Hi Clayarters,
I tried to make big structured slabs (about 2,75 in.), which should imitate
stone slabs.
But in firing they get a few short, thin radial cracks. If not in bisque
firing, then in stoneware firing.
I guess it happens, because the slab can't move on the kiln shelf in the
firing. Is that right?
How can I avoid these cracks?
They even appear in smaller slabs, too.
I hope, someone can give me a hint.
Greetings from Germany
Petra
Craig Clark on tue 11 jan 05
Petra, I'm assuming that the dimension that you give is for the
thickness of the slab.....is this correct? If so then a 2 3/4 inch thick
slab is of a thickness that will require slow even drying to help
mitigate the develpment of stresses that will manifest themselves in
cracks during the firing.
Another thing to be concerned with is the manner in which you make
the slabs. If the cracks radiate in a similar direction then there is a
good chance that when you made the slabs there was a compressive force
applied in only one direction which resulted in an uneven streching of
the clay particles which results in the type of cracking that you have
described. Try it again with the same (roller?) and the turn the clay
ninety degress and run int through again. This will help to mitigate the
problem.
Additionally, the type of clay body that you are using is important
when doing large slab type contructions. I would suggest that you use a
body with a goodly amount of "tooth" in it such as grog or kyanite.
When you fire the slabs keep in mind a few things that have been
mentioned on the list before. As Snail Scott so aptly put it
"......water kills clay....." in that it is the explosive force of
bonded and unbonded water molecules changing to vapor in confined spaces
with no way out the will destroy a piece. You need to go through water
burning very, very slowly. At least overnight on pieces that are this
thick. Candle your kiln for a good twelve hours to be sure that as much
of the water as possible has been released gradually. From what I have
read the intial phase of this occurs with the unbonded water up through
500 degrees F. After that the bonded water is finally released around
800 degrees F. There are more definitive sources for the specifics of
this phenomenon than I on this list.
It is also a good idea to sprinkle grog on your shelf to enable a
heavy piece of slab to move while firing, keeping in mind not to span
the separation between shelves if possible (this will help to minimize
warpage.) The grog acts like a bunch of little ball bearings which take
some or the strain off the piece as it shrinks in firing.
Hope this helps
Craig Dunn Clark
619 East 11 1/2 st
Houston, Texas 77008
(713)861-2083
mudman@hal-pc.org
>
>
>
kathy_chamberlin on wed 12 jan 05
I work with large slabs of clay. Thin to thick but nothing in the
measurement range of 2.75" inches, maybe you are indicating metric
measurement? I am drying for at least 3 weeks with a .25"inch
thickness slab. I'm using canvas to lay slabs of clay out on, canvas
is a very organic and a very complimentary material when working with
clay. I also use laminated boards and bricks to keep the clay "down"
so not to warp and cover with plastic. Visit my website
http://www.aspenceramicstudio.com/ my current work is all slab work.
My best, Kathy Chamberlin
Ivor and Olive Lewis on wed 12 jan 05
Dear Petra Hahn,
I think people will need to know something about your clay before they
can give any advice. Are you mixing your own or using a commercial
product ?
Best regards,Ivor Lewis.
Redhill,
S. Australia.
Paul Gerhold on wed 12 jan 05
Petra,
One additional thing to try. As you roll out the slabs occasionally compress
the outer edges into the center. If possible ,also, do this as a last step
after finishing making the slab. This compression will help mitigate the radial
stress caused by the process of rolling. Also make sure you are rolling in as
many directions as possible since this also reduces the stress forces.
Paul
Petra Hahn on wed 12 jan 05
Hi all,
thanks for your numerous hints, I'll try them all.
After drying the slab was still ok, I dried it very slowly between layers of
newspaper and turned it a few times.
The slabs I try to make are as big as my kiln, about 70 cm, so it's not
possible to surround it with stacks.
I'll let you know, if it works next time.
many greetings
Petra
Vince Pitelka on wed 12 jan 05
Big flat slabs crack for one reason before firing - uneven drying.
The challenge is to dry evenly in the center and the edges. You can wrap the
edges and leave the center partially exposed, allowing the moisture to
evaporate from the center, wicking in from the edges. You can also place
the slab on a slatted rack or a wire rack, allowing evaporation from all
surfaces. It is still a good idea to slow down evaoration from the outside
edge, because that will often cause problems. It is a simple matter to cut
strips of plastic wrap, and smooth them over the outer edges of the slab.
Also, cover the whole slab with a piece of cloth, like an old bedsheet, and
then a sheet of plastic (garbage bag or dry-cleaner bag) with lots of small
holes punched in it. That will allow slow, overall drying, because the
space within the bag is humidified by the moisture in the slab, so the whole
thing dries at the same rate. This works well on all kinds of large clay
forms.
Big flat slabs crack in the firing for two possible reasons - uneven heating
and uneven cooling.
My apologies for contradicting the people who suggested the bed of sand or
grog beneath the slab, but this is important. Placing a large clay piece on
a bed of sand or grog should only be done in glaze firing, where there is
serious shrinkage taking place. It should never be done in bisque firing,
because movement is not the problem. Shrinkage from bone-dry to
bisque-fired is usually almost non-existant, whereas the real problem is
getting even distribution of temperature change all around the piece. A bed
of sand or grog insulates the bottom of the piece from temperature change,
and that is a serious problem.
Large slab forms should be bisque-fired standing on edge, or if that is not
practical, they should be fired on clay wads, even raised up on short posts.
Either way allows circulation of temperature and atmosphere around the
piece. Keep in mind that this is important both in heating and cooling the
kiln.
Good luck -
- Vince
Vince Pitelka
Appalachian Center for Craft, Tennessee Technological University
Smithville TN 37166, 615/597-6801 x111
vpitelka@dtccom.net, wpitelka@tntech.edu
http://iweb.tntech.edu/wpitelka/
http://www.tntech.edu/craftcenter/
| |
|
